1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus, and in particular, to a fixing device and an image forming apparatus capable of fixing an image on a recording medium using a fixing roller heated by an electromagnetic heat induction system.
2. Discussion of the Background Art
Conventionally, an image forming apparatus, such as a copier, a facsimile machine, a printer, a duplicator, etc., produces copies or printed material by fixing a non-fixed image that is transferred and carried on a recording medium with heat of a fixing device.
Specifically, the recording medium carrying the non-fixed image is pinched and conveyed through a fixing nip created between a fixing member, such as a fixing roller, a fixing belt, etc., and a pressing roller that presses against the fixing member. Thus, developer, such as toner, etc., constituting the non-fixed image is fused and softened to penetrate into the recording medium, so that the image is fixed thereon.
When the fixing member does not reach a prescribed level of temperature needed to fix the image, for example, at a start time point, the image forming apparatus executes a pre-heat process so that the fixing member reaches the prescribed level. Recently, a system capable of quickly heating a fixing member without executing the pre-heat process at the start time point has been proposed, for example, as shown in Japanese Patent Application Laid Open No. 2004-109931 (JP-2004-109931-A).
Specifically, the fixing member includes a thin-walled roller or belt made of resin or rubber to achieve a low heat capacity and to quickly execute heating. Further, such a conventional image forming apparatus includes a magnetic flux generating device in the vicinity of the fixing member to generate a magnetic flux and induces heat in the fixing member using the magnetic flux. Thus, the fixing member is effectively heated within a short time period.
In an image forming apparatus having such a low capacity fixing member, the fixing nip has a large curvature to provide good separation of the recording medium from the fixing member when the recording medium passes through the fixing nip. As a consequence, however, the recording medium is likely to be curled upon existing the fixing nip.
To resolve such a problem, a system having a pair of de-curling rollers that creates a correction nip having a reverse curvature to that of the fixing nip therebetween has been proposed, for example, as described in Japanese Patent Application Laid Open No. 2006-23427 (JP-2006-23427-A).
Specifically, such a system includes a fixing nip that fixes a non-fixed image with heat onto a recording medium, the correction nip that corrects a curl of the recording medium passing through the fixing nip, and a de-curling mechanism that has a guide downstream of the correction nip and adjusts a conveyance direction of the recording medium that passes through the correction nip. The guide changes the conveyance direction of the recording medium when the recording medium is passing through the correction nip to adjust an amount of correction of the curl in accordance with characteristics or basic weight of the recording medium. Thus, the amount of correction of the curl can be adjusted as the characteristics or the basic weight change.
However, in both of the above-mentioned conventional image forming apparatuses, heat expansion of the de-curling rollers is neglected even if the temperature of the rollers changes as a result of being heated by the recording medium heated in the fixing nip. Accordingly, due to changes by heat expansion of the de-curling rollers, a line velocity sometimes changes in the correction nip. As a result, differences in line velocity between the fixing nip and the correction nip cannot be kept constant, thereby causing sheet jam.